1. Field of the Invention
The present invention relates to an image recording apparatus using a laser beam and suited for an electrophotographic color copying machine and a laser printer.
2. Description of the Prior Art
We have proposed a laser recording apparatus using a deflector, as disclosed in Japanese Patent Laid-Open No. 63-14117. In the prior art, a rotary polygon mirror is used to deflect and scan a laser beam. Thus, the polygonal body having considerable mass is rotated at a high speed so that the image recording apparatus of the prior art is highly stable and hardy influenced by the oscillations coming from a recording unit. The rotary polygon mirror is fixed directly and firmly to the optical base of an image pickup system so that the scanning by the laser beam can be stabilized.
In other words, it has been common knowledge in the prior art that the optical deflector is firmly fixed to the optical base. The following structure is disclosed in Japanese Patent Laid-Open No. 63-14117.
FIG. 6 is a top plan view showing the structure of the beam scanning optical system of the above-specified laser recording apparatus.
A laser beam emitted from a semiconductor laser 31 has its shape corrected by a collimator lens 32 and is guided through a cylindrical lens 33 until it is introduced into a deflector 300 by a reflecting mirror 41. The deflector 300 deflects the laser beam in a predetermined direction at a predetermined speed.
The laser beam thus deflected is focused on an image retainer 11 to form an electrostatic latent image by a scanning lens 42 and a cylindrical lens 36.
The cylindrical lenses 33 and 36 are used to correct the tilt, if any, of the reflecting mirror attached to the deflector 300. Reference numeral 38 designates a reflecting mirror for reflecting the laser beam, when this beam is reflected to the uppermost end, as shown, to a sensor 39 to generate a scan starting signal.
Here, one cylindrical lens 36 may be made of plastics. In case a plastic lens is used, its facial shape can be relatively simply matched with the optimum shape so that the performance of the whole optical system can be improved.
In case, however, the tilt of the reflecting mirror is very small, the aforementioned cylindrical lenses 33 and 36 can be omitted.
The scanning lens 42 is used to focus the laser beam correctly on the surface of the image retainer and to scan the image retainer with the laser beam at a constant speed.
In case the reflecting mirror is oscillated at the intrinsic frequency of the deflector 300, its angle of deflection .theta. is expressed by the following formula: EQU .theta.=A sin .omega.t,
where
A: the maximum angle of deflecting of the reflecting mirror;
.omega.: an angular velocity; and
t: a time.
Therefore, if the spot position of the laser beam is a function X(.theta.) of the angle .theta., the scanning lens 42 is provided with the following characteristics: EQU X(.theta.)=A.f.arcsin (.theta./A),
where f: the focal length of the scanning lens 42.
Then, in case the spot position of the laser beam on the image retainer 11 is expressed as the function X(t) of the time t, this function is expressed in the following form from the above-specified formula: EQU X(t)=A.f..omega.t.
As a result, conversion in uniform motion can be accomplished by the use of that scanning lens 42, as described above. An image quality without any distorsion can be obtained in case the electrostatic latent image is to be formed by the uniform motion.
The deflector 300 to be used in such optical scanning system can be exemplified by the deflector 300 using a deflecting member 331, as shown in FIG. 7.
With reference to FIG. 7, the deflector 300 has an elongated frame 315 having a generally rectangular shape. At a central portion of the frame 315, there is disposed a drive coil 311 which is overlain by a reflecting mirror 312. The reflecting mirror 312 and the drive coil 311 are rotatably supported by ligaments 313a and 313b which are interposed between the reflecting mirror 312 and the frame 315 and between the drive coil 311 and the frame 315, respectively.
Thus, the deflecting member 310 is integrally constructed of the drive coil 311, the reflecting mirror 312 and the rotatably supporting ligaments 313a and 313b.
The frame 315 may be made of a material allowing an easy etching and having a large elastic coefficient, such as crystals of rock crystal or quartz, or glass. In case the rock crystal is used, the frame 315 desirably has a thickness of about 0.1 mm to 0.5 mm.
The working means for forming the deflecting member 310 in the frame 315 is usually the photolithography and the etching technique for the fine working. The surface of the deflecting member 310 thus etched is plated with chromium and then silver so as to reduce the electric resistance.
In case, on the other hand, a semiconductor laser is used as a light source, the reflecting mirror 312 is plated with gold, silver or aluminum so as to raise its reflectivity. Moreover, the reflecting mirror 312 can be coated, after plated, with a protecting film such as SiO or SiO.sub.2 so as to protect its flaw or oxidization.
The reflecting mirror 312 has the following shape selected. Specifically, the laser beam has an elliptical shape long sideways after it has passed through the collimator lens 32 and the cylindrical lens 33. The reflecting mirror 312 to be used may be elongated in the main scanning direction.
In case, moreover, the reflecting mirror 312 is oscillated at a high speed, an air resistance raises a problem, and the reflecting mirror 312 may be advantageous if it has an elliptical shape long sideways, as shown in FIG. 7.
The larger length of the reflecting mirror 312 is different depending upon the focal length of the scanning lens 42, the diameter of the beam spot to be focused on the image retainer 11, or the scanning width on the image retainer 11 and is desirably 4 to 10 mm according to the experiments.
The drive frequency f for oscillating the deflecting member 310 is desirably set either equal to the intrinsic frequency f.sub.0 of the deflecting member 310 or within the range defined by the following formula, so as to suppress the input current to the drive coil 311: EQU F=f.sub.0 .+-.f.sub.0 /Q
where Q: the sharpness of resonance of the resonant characteristics.
By using a deflector using the deflecting member, according to the present invention, it is possible to realize a laser recording apparatus which has far higher reliability and higher image quality than those of the prior art. The advantages of the laser recording apparatus of the present invention are as follows:
At first, the deflector is so small-sized that the laser recording apparatus can be more small-sized than that using the rotary polygon mirror Since no motor is used as a rotating drive source, stable oscillations for the deflections can be realized at all times without any noise even for a high-speed scanning.
Secondly, it is possible to realize a small-sized deflector which can be at a higher scanning speed and with a smaller angle of deflection than that using a mechanical oscillating mirror.
Thirdly, the deflecting member is formed by the etching treatment so that it has a high accuracy but no dispersion in its quality. Since, moreover, the ligaments are made of the material having a high elastic coefficient, it is less fatigued than a metal rod being oscillated by a mechanical oscillating mirror so that stable operations can be expected for a long time.
Thus, the laser recording apparatus according to the present invention can have a very high reliability so that it can provide a highly reliable recording apparatus.
Fourthly, since the deflecting member is molded into one piece, it is possible to establish a large angle of deflection and a high intrinsic number of oscillations. Therefore, the deflecting member is suited for an apparatus which uses a large recording-paper size and aims at a higher recording.
Fifthly, since the reflecting mirror face of the deflector is not so large as the beam spot, the optical scattering on the reflecting face less influences. Since the deflecting member is molded into one piece, the mirror is stably oscillated even if the surrounding temperature and the environmental conditions change. Thus, a regular beam scanning can be accomplished. As a result, a satisfactory final image can always be obtained even for a high-speed recording operation.
The proposed apparatus has the excellent advantages described above but is not resistant to the vibrations and impacts applied from the outside to the beam scanning optical system. Thus, there arises a problem that the quality of the image obtained by the scanning is dropped by the influences of the oscillations and the impacts.